This invention relates to a method for forming photographic images by silver salt diffusion transfer.
The principle of silver salt diffusion transfer process is described in U.S. Pat. No. 2,352,014.
The silver salt diffusion transfer process generally uses a photosensitive material and an image-receiving material. The photosensitive material which has a silver halide base emulsion layer is exposed imagewise and then combined with the image-receiving material such that the exposed emulsion layer faces the receiving surface of the latter. The assembly is immersed in a developer solution, squeezed of excess solution, and allowed to stand for a little while. Those portions of the silver halide in the photosensitive material which have been fully exposed are developed by the action of the developer. The unexposed silver halide portions are solubilized by the action of a silver halide solvent, diffused into the image-receiving material, and reduced in the presence of physical development nuclei into silver to form a positive image.
This process utilizes an alkali-containing developer, which would be hazardous to the hands and cloths and cause corrosion of a developing machine. Handling of developing chemicals and maintenance of developer solution require some skill, and the process is cumbersome as a whole.
A number of methods for forming positive images by heating have been proposed to overcome these drawbacks. It is, however, fairly difficult to solubilize a silver halide with a silver halide solvent solely by applying heat. Also, it takes a time to transfer the solubilized silver salt onto an image-receiving material with the aid of heat only. A hydrophilic thermal solvent is used to promote transfer of a mobile silver salt as described in Japanese Patent Application Kokai No. 60-194448.
The silver halide solvents described in these methods have problems of suppressing development, discoloring a silver image on an image-receiving material, and so on.